linux-kernel - Re: [PATCH v2 03/13] mm: Add generic p?d

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Message-ID: <e0c7fc0c-7924-1106-a7a3-fc12136b7b82@arm.com>
Date:   Thu, 21 Feb 2019 17:16:46 +0000
From:   Steven Price <steven.price@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        James Morse <james.morse@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v2 03/13] mm: Add generic p?d_large() macros

On 21/02/2019 14:57, Kirill A. Shutemov wrote:
> On Thu, Feb 21, 2019 at 02:46:18PM +0000, Steven Price wrote:
>> On 21/02/2019 14:28, Kirill A. Shutemov wrote:
>>> On Thu, Feb 21, 2019 at 11:34:52AM +0000, Steven Price wrote:
>>>> From: James Morse <james.morse@....com>
>>>>
>>>> Exposing the pud/pgd levels of the page tables to walk_page_range() means
>>>> we may come across the exotic large mappings that come with large areas
>>>> of contiguous memory (such as the kernel's linear map).
>>>>
>>>> For architectures that don't provide p?d_large() macros, provided a
>>>> does nothing default.
>>>
>>> Nak, sorry.
>>>
>>> Power will get broken by the patch. It has pmd_large() inline function,
>>> that will be overwritten by the define from this patch.
>>>
>>> I believe it requires more ground work on arch side in general.
>>> All architectures that has huge page support has to provide these helpers
>>> (and matching defines) before you can use it in a generic code.
>>
>> Sorry about that, I had compile tested on power, but obviously not the
>> right config to actually see the breakage.
> 
> I don't think you'll catch it at compile-time. It would silently override
> the helper with always-false.

Ah, that might explain why I missed it.

>> I'll do some grepping - hopefully this is just a case of exposing the
>> functions/defines that already exist for those architectures.
> 
> I see the same type of breakage on s390 and sparc.
> 
>> Note that in terms of the new page walking code, these new defines are
>> only used when walking a page table without a VMA (which isn't currently
>> done), so architectures which don't use p?d_large currently will work
>> fine with the generic versions. They only need to provide meaningful
>> definitions when switching to use the walk-without-a-VMA functionality.
> 
> How other architectures would know that they need to provide the helpers
> to get walk-without-a-VMA functionality? This looks very fragile to me.

Yes, you've got a good point there. This would apply to the p?d_large
macros as well - any arch which (inadvertently) uses the generic version
is likely to be fragile/broken.

I think probably the best option here is to scrap the generic versions
altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which
would enable the new functionality to those arches that opt-in. Do you
think this would be less fragile?

Thanks,

Steve